lib.js

/* flow-typed/lib.js */

type JSXHelper<T> = Class<React$Component<{}, T, any>>;

// $JSXIntrinsics is special and magic.
// This declares the types for `div` and `span`
type $JSXIntrinsics = {
  div: JSXHelper<{id: string}>,
  span: JSXHelper<{id: string, class: string}>,
};

main.js

// @flow

let myDiv = <div id={42} />;

output

> flow version
Flow, a static type checker for JavaScript, version 0.36.0
> flow check
main.js:3
  3: let myDiv = <div id={42} />;
                 ^^^^^^^^^^^^^^^ React element `div`
  3: let myDiv = <div id={42} />;
                          ^^ number. This type is incompatible with
  6:   div: JSXHelper<{id: string}>,
                           ^^^^^^ string. See lib: flow-typed/lib.js:6


Found 1 error